Building external installations
New bases are build from the Geoscape. There are tree other installations that can be build, but you will require multiple operational bases to be able to build a lot of them and assist your bases to defend the single remote part of Earth.
Building new bases
A new base can be build anywhere around the world. Unlike your first base that comes fully operationnal with a lot of buildings a new base comes only with the entrance. You will have to build all other facilities so getting an operational base will require a lot of funds and time.
Radars will extends your detection zone. The external radars are about half as efficient as the one you can build inside a base but will still give a good detection range.
Building missile sites
A missile site can hold two missile batteries. These batteries and missiles are generously provided by the United Nations so you don't have to care about it once the site is build.
Building UFO yards
An UFO yard is required to store captured UFO's instead of selling them once recoverd. You need more than one base to build it and it will store multiple UFO's.
The closer to a base it is, the easier it is for the sciencists to study them.